mahalanobis_distance_impl.hpp

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#ifndef MLPACK_CORE_METRICS_MAHALANOBIS_DISTANCE_IMPL_HPP
#define MLPACK_CORE_METRICS_MAHALANOBIS_DISTANCE_IMPL_HPP

#include "mahalanobis_distance.hpp"

namespace mlpack {
namespace metric {

template<>
template<typename VecTypeA, typename VecTypeB>
double MahalanobisDistance<false>::Evaluate(const VecTypeA& a,
                                            const VecTypeB& b)
{
  arma::vec m = (a - b);
  arma::mat out = trans(m) * covariance * m; // 1x1
  return out[0];
}
template<>
template<typename VecTypeA, typename VecTypeB>
double MahalanobisDistance<true>::Evaluate(const VecTypeA& a,
                                           const VecTypeB& b)
{
  // Check if covariance matrix has been initialized.
  if (covariance.n_rows == 0)
    covariance = arma::eye<arma::mat>(a.n_elem, a.n_elem);

  arma::vec m = (a - b);
  arma::mat out = trans(m) * covariance * m; // 1x1;
  return sqrt(out[0]);
}

// Serialize the Mahalanobis distance.
template<bool TakeRoot>
template<typename Archive>
void MahalanobisDistance<TakeRoot>::serialize(Archive& ar,
                                              const uint32_t /* version */)
{
  ar(CEREAL_NVP(covariance));
}

} // namespace metric
} // namespace mlpack

#endif